High efficiency paint arrestance filter

ABSTRACT

Various examples of the inventive subject matter include a method and corresponding system for fabricating a combination paint arrestance filter for use in the painting industry. Embodiments of the system include a single-stage combination filter having a number of first layer media pockets and a number of second layer media pockets arranged downstream of the first layer pockets. An opening of each of the first layer media pockets is arranged in parallel with one another to receive an incoming airflow into the single-stage combination filter. Each of the second layer media pockets is at least 99.97% efficient at removing 0.3 micron and larger particles at a face velocity on the filter of about 37 meters per minute.

RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.14/290,522, filed May 29, 2014, which is a divisional of U.S. patentapplication Ser. No. 13/187,216, filed on Jul. 20, 2011, the benefit ofpriority of which is claimed hereby, and of which is incorporated byreference herein in their entireties.

BACKGROUND

The local atmospheric environment (i.e., air) contains a vast array ofnatural and man-made particles. These particles have a wide particlesize distribution from being visible to the naked eye tosub-microscopic. Particle sizes described herein are generally definedby a given aerodynamic size, expressed in micrometers (i.e., microns(μm)) where one micron is one-one millionth of a meter or approximately39.4 micro-inches (about 1/25,000 of an inch). Depending upon lightingand contrast conditions, visually detectable particles are approximately50 microns or larger. The numbers of sub-micron particles in air is fargreater than larger particles because of the mass of the particles,respectively. Settling velocities for small particles, in either stillair or in an air stream (moving air), are far greater than largeparticles. An air stream can suspend smaller particles for longerperiods of time than larger particles.

Heating, ventilation, and air conditioning (HVAC) systems are made ofcombinations of duct work, fans, heaters, coolers, humidifiers, andfilters that condition and deliver the air to occupied spaces providingcomfort or the necessary environment in which to complete certain tasks.HVAC systems are common in all building structures. In most parts of theworld, regulations govern HVAC systems depending on the function of thespace that is being serviced. For example, filtration systems forpharmaceutical manufacturers, hospitals, and high-tech manufacturers canbe complicated and multi-staged and may require a particular filterefficiency for a given face velocity of air at an output of the filter.Frequently, the regulations may also require a particular minimum filterefficiency for a given particle size.

Multi-staged filtration systems are filters placed in series with thelowest efficiency filter placed first in the series and the highestefficiency filter placed last in the series. “Last in the series” meansclosest to an output of the multi-staged filtration systems, such asjust before the airflow through the filters enters a filtered room. Theseries arrangement is an economical way of filtering air. The lowerefficiency filter entraps the larger sized particle, passing the smallersized particles to the next filter in the series, and so on. In thisrespect, a multi-staged filtration system can be analogized to a sieve(although particles much smaller than the space between filter media maystill be trapped by diffusion mechanisms). Additionally, placing filtersin series allows the lower efficiency filter to act as a pre-filter tothe higher efficiency filter located next in the series. Thus, the lowerefficiency filter can be changed more often, saving the next moreexpensive filter (as filter efficiency increases, the price alsoincreases).

Hospitals, for example, may have a four-stage filtration system withfilters placed in series and placed from low-efficiency tohigh-efficiency, such as may be found in a high-efficiency particulateair (HEPA) filter. The filtering media used are typically fiberscomprised of paper-like material or fiberglass and are highlyrestrictive to airflow. HEPA filters function to trap particles throughthree mechanisms: interception, impaction, and diffusion. Particleinterception occurs when particles in an airstream (i.e., airflowthrough the filter) come within one radius of the filter fibers and aretrapped by the fibers. Particle impaction occurs when particles impactdirectly onto a fiber. Particle diffusion occurs as a result of acollision of particles with gas molecules and accounts for why particlesmuch smaller than the space between filter fibers can be trapped on thefibers. Since, for a given airflow, particle diffusion occurs withincreasing frequency as particle size become smaller (especially whenparticles are smaller than approximately 0.1 μm in size), HEPA filtersare rated by the most penetrating particle size (MPPS). For a given facevelocity of air exiting a HEPA filter, for example, about 37 meters perminute (approximately 120 feet per minute), the combination of thesethree filtering mechanisms means that 0.3 μm particles are the MPPS.Thus, HEPA filters are defined as 99.97% efficient on 0.3 μm particles.

However, as the filters become more efficient the media used to producethe filters becomes denser and, therefore, more restrictive to air flow.The restriction to air flow creates a pressure drop within the filter.The pressure drop increases as the filters become loaded with particlesthat the filters have trapped. To compensate for the pressure drop,medium-efficiency, high-efficiency, and HEPA filters are constructed inways to increase the effective filtering area in a given face size. Thefilters are constructed using pleats and bag pockets to increase theeffective filtering area (e.g., as measured in square centimeters orsquare feet) and thus reducing the resistance to air flow or pressuredrop created by the filter. Reducing pressure drop and increasing filterlife by increasing effective filter area of filter media is paramount tothe efficient design, installation, and operation of any HVAC systembecause the higher pressure drop requires larger fans and motors and,consequently, increased electrical power to run the fans and motors.

The filtering media in HEPA filters are densely pleated to maximizefilter volume and the media packs containing all the filter media aresealed into steel or wooden frames. Even with the dense pleatingtechniques used to produce HEPA filters. HEPA filters still have a highrestriction to airflow as well as a high associated cost. For example, aHEPA filter can produce an initial pressure drop of 300 Pascal(approximately 2.25 millimeters of mercury or 1.2 inches of water) at aface velocity of 37 meters per minute.

Particles trapped on the HEPA filters tend to load on the incoming face(i.e., where the airflow enters the filter) of the filter, as opposed toa depth-loading lofted media. With lofted media, particles can enter themedia and be captured within the maize of filter material that compriseslofted media filters. Consequently, the lofted media allows more air toenter the media and work its way through the filter. Additionally, HEPAfilters are relatively heavy (e.g., about 18 kilograms each(approximately 40 pounds)), each and need a separate filter holdingarrangement designed to hold and seal the filters in the air stream.

Filter holding arrangements that have been used in various industries inthe past simply cannot accommodate the newest demands and regulationsfor filtered air applications. For example, in the case of the aerospaceindustry, a wall of filters required for painting aircraft can requirefilter holding arrangements covering substantially all wall surfaces ofan airplane-hangar sized structure. Consequently, due to the newerregulations requiring HEPA filtration for paint booths, the aerospaceand other painting industries are forced to completely rebuild thefilter holding arrangements currently in use to meet the new regulationsrequiring HEPA filters. Additionally, new fans and motors powerfulenough to create sufficient airflow through the HEPA filters arerequired. Therefore, what is needed is a filter that can be retrofitinto existing filter holding arrangements, using existing fans andmotors, and still meet advanced filtration requirements imposed byvarious governmental agencies.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of a filter wall used in paintbooths in the aerospace and other industries in which paint booths areused;

FIG. 2A shows a side elevation view of a filter holder monitoring framethat may be used with the filter wall of FIG. 1;

FIG. 2B shows an isometric view of a filter holder monitoring frame thatmay be used with the filter wall of FIG. 1;

FIG. 3 shows an example of a three-stage filter system that may be usedwith the filter wall of FIG. 1 or the monitoring frame of FIG. 2A or 2B:

FIG. 4A shows an example of a four-stage filter system that may be usedto comply with, for example, advanced exhaust pollution requirements;

FIG. 4B shows an example of an combination filter that may be used tocomply with, for example, advanced exhaust pollution requirements andthat further may be used with the filter wall of FIG. 1 or themonitoring frame of FIG. 2A or 2B:

FIG. 5 is a schematic diagram of an example test apparatus used fortesting filter efficiency;

FIG. 6 is an example graph showing fractional efficiency as a functionof particle diameter in testing the combination filter of FIG. 4B;

FIG. 7 is an example graph showing pressure drop as a function of massof paint in testing the combination filter of FIG. 4B:

FIG. 8 is an example method for constructing the combination filter ofFIG. 4B;

FIG. 9A shows a front elevational view of the combination filterconstructed in accordance with the method of FIG. 8:

FIG. 9B shows an example cross-sectional view of one of the pockets ofthe combination filter of FIG. 9A;

FIGS. 9C through 9E show various views of the example combination filterof FIG. 9A; and

FIGS. 10A and 10B show example alternative fabrication techniques forconstructing the filter of FIG. 9A.

DETAILED DESCRIPTION

The description that follows includes illustrative systems, methods,techniques, and fabrication sequences that embody at least portions ofthe inventive subject matter. In the following description, for purposesof explanation, numerous specific details are set forth in order toprovide an understanding of various embodiments of the inventive subjectmatter. It will be evident, however, to those skilled in the art thatvarious embodiments of the inventive subject matter may be practicedwithout these specific details. Further, well-known structures, methods,fabrication technologies, and techniques have not been shown in detail.

As used herein, the term “or” may be construed in an inclusive orexclusive sense. Similarly, the term “exemplary” is construed merely tomean an example of something or an exemplar and not necessarily apreferred or ideal means of accomplishing a goal. Additionally, althoughvarious exemplary embodiments discussed below focus on a combinationfilter for use in, for example, the industrial and aerospace paintingindustries, the embodiments are merely given for clarity in disclosure.As an introduction to the subject, a few embodiments will be describedbriefly and generally in the following paragraphs, and then a moredetailed description, with reference to the figures, will ensue.

In various embodiments, a single-stage combination filter is providedthat has a number of first layer media pockets and a number of secondlayer media pockets arranged downstream of the first layer pockets. Anopening of each of the first layer media pockets is arranged in parallelwith one another to receive an incoming airflow into the single-stagecombination filter. Each of the second layer media pockets is at least99.97% efficient at removing 0.3 micron and larger particles at a facevelocity on the filter of about 37 meters per minute.

In another embodiment, a method of forming a combination filter isprovided. The method includes cutting a first layer media material,cutting a second layer HEPA media material and alternately interleavingthe first layer media and the second layer HEPA media to form pairs ofthe first layer media and the second layer media. The pairs of the firstlayer media and the second layer media are then folded with the firstlayer media being arranged inside of the second layer HEPA media. Sidesof the folded pairs of the first layer media and the second layer mediaare then sealed. A rigid material is inserted into an open edge of thefolded pairs of the first layer media and the second layer media,forming a pocket assembly. A number of the pocket assemblies is insertedin parallel to one another into a header assembly.

In another embodiment, a three-stage filter for use in the aerospaceindustry is provided. The three-stage filter includes a first-stagepre-filter, a second-stage medium-efficiency filter downstream of thefirst-stage pre-filter, and a third-stage combination filter downstreamof the second-stage medium-efficiency filter. The third-stagecombination filter has a number of first layer media pockets to receivean incoming airflow with a number of second layer HEPA media pocketsdownstream of the plurality of first layer media pockets.

Paint booths are rooms prepared with a specific purpose of creating anenvironment within for applying coating (e.g., paint) to a manufacturedpart. As noted above, a paint booth can be the size of an airplanehangar. However, the environment of the paint booth can still becontrolled since it is a confined space.

Generally, a paint booth contains two HVAC systems. One HVAC system isto supply fresh, conditioned air into the booth. The other HVAC systemis to exhaust overspray air out of the booth. Overspray is paint (orother coatings) that did not adhere to the manufactured part beingpainted or coated.

For example, inside the booth, liquid paint is delivered to a paint gunwhere the paint is atomized and delivered through the gun in a fog ormist-like state. Some of the paint fog attaches to the part and some ofthe paint stays suspended in the air, thus creating overspray. Theoverspray is then exhausted out of the paint booth through a filtrationsystem. As discussed in more detail, below, the paint fog of overspraycontains a wide range of particle sizes and therefore multi-stagedfiltration systems are designed for these systems.

With reference to now to FIG. 1, a schematic representation of a filterwall 100 is shown that is used in paint booths in the aerospace andother industries in which paint booths are used. The filter wall 100 isshown to include a multi-stage filter holder frame 101, a number offiller strips 103 to fill spaces between the filter wall 100 andadjoining side walls (not shown), and an optional filter holdermonitoring frame 105. An arrow 107 also indicates a direction ofairflow. As indicated by the arrow 107, the direction of airflow is froman inside area 109 of the paint booth to an exhaust area 111. Theexhaust area 111 may contain a plenum and ductwork (not shown) to directfiltered air out of the room or building housing the paint booth.

The multi-stage filter holder frame 101 can be fabricated to hold one ormore filters of a particular size in series. For example, themulti-stage filter holder frame 101 may accommodate a standard sizedfilter having outside dimension of about 50.8 cm by 50.8 cm(approximately 20 inches by 20 inches) filter. The filler strips 103 maybe arranged to fill in any space remaining between integral numbers ofthe filter holder frames and adjoining side walls and a ceiling of thepaint booth.

Referring now to FIGS. 2A and 2B concurrently, the filter holdermonitoring frame 105 that may be used with the filter wall of FIG. 1 isshown. The filter holder monitoring frame 105 of FIGS. 2A and 2B isarranged to hold a three-stage filter in series. With direct referenceto the side elevational view of FIG. 2A, the filter holder monitoringframe 105 is shown to include a first area 201A for holding afirst-stage filter 201B, a second area 203A for holding a second-stagefilter 203B, and a third area 205A for holding a third-stage filter205B.

Within the filter holder monitoring frame 105 of FIG. 2A, thefirst-stage filter 201B is a pre-filter and may be selected to be alow-efficiency flat pad filter. The second-stage filter 203B may be amedium-efficiency multi-layered panel. As shown in FIG. 2A and describedin more detail below, the third-stage filter 205B is referred to in theindustry as a multi-pocketed bag filter. The multi-pocketed bag filterhangs through and extends beyond the filter holder monitoring frame 105and into the exhaust area 111 (FIG. 1) or a plenum (not shown) withinthe exhaust area 111.

Multi-pocketed bag filters are typically 30 cm (approximately 12 inches)to 50 cm (approximately 20 inches) in length from the frame of the bagfilter to a trailing or downstream edge of the bags. Since themulti-pocketed bag filter is comprised of a number of individual pocketswithin the bag, an overall surface area presented to the airflow is muchgreater than an area of the filter holder monitoring frame 105 byitself. As a result of the increased surface area, the pressure dropthrough the bag filter is reduced as compared with a standard flatfilter design. Construction of various embodiments of multi-pocketed bagfilters is discussed in more detail, below.

The filter holder monitoring frame 105 of FIG. 2A is further shown toinclude a number of tube fittings 209 and a number of pressure gauges207. The tube fittings 209 place the pressure gauges in direct fluidcommunication with pressure between the first area 201A, the second area203A, and the third area 205A of the filter holder monitoring frame 105.Consequently, the pressure gauges 207 can monitor a pressure drop acrosseach of the filters 201B, 203B, 205B. Since the pressure drop acrosseach of the filters is being monitored, a user of the paint booth canreadily determine when each of the filters is loaded with paint, asdiscussed above, and can change out one or more of the three filters asneeded to ensure continued filtering efficiency without an excessivepressure drop across the filter.

For example, the center one of the pressure gauges 207 is in fluidcommunication, through two of the tube fittings 209, with the secondarea 203A and the third area 205A. Since the center one of the pressuregauges 207 measures an upstream pressure within the second area 203A anda downstream pressure in the third area 205A, the pressure gaugetherefore measures the pressure drop across the second-stage filter203B. Similarly, the first (i.e., the one on the left) of the pressuregauges 207 is in fluid communication with ambient pressure at a point211 inside the paint booth and with the second area 203A through thefirst on the tube fittings 209 coupled to the second area 203A. Thus,the first of the pressure gauges 207 measures an upstream pressure atthe point 211 within the paint booth and a downstream pressure in thesecond area 203A. The pressure gauge therefore measures the pressuredrop across the first-stage filter 201B.

Within the United States, both Federal and state-level agencies, such asthe Environmental Protection Agency (EPA), create regulations to controla level of filtration required on exhaust systems (e.g., within theexhaust area 111 of FIG. 1) of paint booths. These regulations can bedifferent and more-or-less strict depending upon the paint formulation.For example, if the paint used contains chromates, the regulations arefar more restrictive. The regulations may also establish the airvelocity (e.g., a face velocity at a downstream side of the filter) andflow rates through the filters. For example, in the United States, theair velocity and air flow rate are defined in feet per minute (FPM) andcubic feet per minute (CFM), respectively.

Certain industries may use paints, such as chromate additives, comprisedof more volatile chemicals for wear, durability, and adhesion purposes.Thus, more stringent regulations typically apply to these industries.For example, the aerospace industry uses many paint formulations whichcontain hazardous pollutants such as hexavalent chromium. To account forthe more hazardous pollutants, on Sep. 1, 1995, the United StatesEnvironmental Protection Agency published the final aerospace NationalEmission Standards for Hazardous Air Pollutants (NESHAP, also referredto as EPA Test Method 319), published under 40 C.F.R. §63. Subpart GG.The rule regulates emissions of Hazardous Air Pollutants (HAP) fromaerospace manufacturing and rework facilities (including maintenance andrepair) that are a major source of HAP.

The NESHAP rule addresses the quality of the exhausted air fromaerospace paint booths by establishing a test method and minimumefficiencies on a range of particle sizes of a wet and a dry filterchallenge agent. In addition, the NESHAP rule requires a three-stagefiltration system. Three-stage filtration systems are readily availablefrom several filter manufacturers for this application.

With reference now to FIG. 3, an example of a three-stage filter systemthat may be used with the filter wall of FIG. 1 or the monitoring frameof FIGS. 2A and 2B is shown. With the proper level of filtration at therequired flow rate, the three-stage filter system of FIG. 3 may be incompliance with the NESHAP rule. Although four filters are shown, askilled artisan will immediately recognize that the second-stage filter203B of FIG. 2A may be implemented by either a medium-efficiency flatfilter 203B₁ or, optionally, by a medium-efficiency bag filter 203B₂ ofFIG. 3. As indicated in FIG. 3, an overall depth D₁ of the three-stagefilter system is dependent upon which filter is selected for the secondstage. The medium-efficiency flat filter 203B₁ has a depth of D₂ whereasthe medium-efficiency bag filter 203B₂ has a depth of D₃. Therefore, ifthe medium-efficiency bag filter 203B₂ is selected as the second-stagefilter, an overall depth D₁ of the three-stage filter system increasesdue to the increased depth (i.e., the difference of D₃−D₂) of themedium-efficiency bag filter 203B₂ since the medium-efficiency bagfilter 203B₂ should not touch the third-stage filter 205B or thepressure drop across the third-stage filter 205B would otherwiseincrease. Each of the individual filter stages may use readily-availablecommercial filters as known independently to one of skill in the art.

In the United States, the State of California increased the stringencybeyond what is required by the NESHAP rule. The California State EPAincreased the final filtration requirement to HEPA filtration quality.This additional requirement for HEPA filtration quality generally meansthat an additional HEPA frame system must be installed behind thethree-stage frame, discussed above, to hold and seal the added HEPAfilters. In addition, as also discussed above, the increased pressuredrop due to the HEPA filter requires an increase in motor power tooperate against the increased pressure drop, larger fans to move theair, and increased electrical power to compensate for the increasedresistance to air flow due to the higher pressure drop. Also the paintbooths must be designed with a larger foot print to allow for theinstallation and maintenance of the HEPA filters. The HEPA requirementincreases the initial cost, on-going maintenance, and on-going energyrequirements needed to operate the paint booth with HEPA filters. Thesepaint booths range in size from small, for manufactured parts, to paintbooths large enough to accommodate full-sized aircraft.

FIG. 4A shows an example of a four-stage filter system that may be usedto comply with, for example, advanced exhaust pollution requirementsunder the State of California rule. The four-stage filter system of FIG.4A utilizes the same or similar design to the three-stage filter systemof FIG. 3 but also incorporates a HEPA filter 401 as the final-stage inthe system. The HEPA filter 401 may be, for example, a commerciallyavailable HEPA filter from a number of suppliers, such as MICROGUARD®,manufactured by AirGuard of Louisville, Ky. USA. However, as will beimmediately recognizable to a person of skill in the art and asdiscussed above, the four-stage filter system of FIG. 4A cannotphysically be implemented into existing paint booths due to therequirement for a fourth, and much larger, final stage filter—the HEPAfilter 401.

Referring to FIG. 4B, an exemplary embodiment of an combination filterthat may be used to comply with, for example, advanced exhaust pollutionrequirements and that further may be used with the filter wall of FIG. 1or the monitoring frame of FIGS. 2A and 2B is shown. Although thecombination filter of FIG. 4B appears similar to the three-stage filtersystem of FIG. 3, FIG. 4B is actually a four-stage filter system. Acombination filter 431 effectively incorporates components from both themedium-efficiency bag filter 203B₂ of FIG. 3 and the HEPA filter 401 ofFIG. 4B into a single-stage. Consequently, the combination filter 431 isa single-stage filter that meets the HEPA requirement under the State ofCalifornia rules, has a form-factor to physically fit within thethree-stage aerospace parameter required under NESHAP, and has a muchlower initial pressure drop than standard HEPA filters.

In an embodiment, the combination filter 431 utilizes a combination oftwo layers of electrostatic media combined as if it were a single media.The first layer of material of the combination filter 431 isapproximately 90% efficient and pre-filters the second layer. The firstlayer material may be an electrostatic media. Such an electrostaticmedia is commercially available from, for example, Kimberly ClarkCorporation of Neenah, Wis., USA, and is referred to as “95 SPFiltration Media.”

The second layer of the combination filter 431, which is approximately99.97% efficient, is a lofted electrostatic HEPA filtration media and,in some embodiments, mechanically coupled to the first layer. Onecommercial source for the lofted electrostatic HEPA filtration media isavailable from Hollingsworth & Vose of Cumbria, England, UK, under thename TECHNOSTAT®. TECHNOSTAT® is a blended, nonwoven synthetic fiberattached to a polypropylene spun-bonded scrim.

To verify efficiency and loading characteristics of the combinationfilter 431, test samples were prepared for testing by LMS Technologies.Inc. (Edina, Minn., USA) in accordance with the EPA Test Method 319testing guidelines. The combination of the two-layer electrostatic mediaallows the three-stage system of FIG. 4B to pass both the Federal NESHAPrequirements and the HEPA emission requirements of the State ofCalifornia with a single filter.

FIG. 5 is a schematic diagram of an exemplary test apparatus 500 usedfor testing filter efficiency. The exemplary test apparatus 500 is shownto include a particle generation section 531 and a filter chamber testsection 551. The particle generation section 531 includes a throttlingvalve 501, an absolute filter 503, a nebulizer 505, a diffusion dryer507, and a charge neutralizer 509.

The throttling valve 501 may be, for example, a mechanical constrictiveor obstructive pneumatic valve or, alternatively, a mass flow meter. Thethrottling valve 501 accepts and controls flow of a gas flow, such asclean dry air (CDA) or a bottled inert-gas, such as nitrogen. The gasflow is then filtered by the absolute filter 503 and enters thenebulizer 505. The nebulizer 505 may be any of a variety of commerciallyavailable nebulizers such as a Collison nebulizer. The nebulizer maycontain a monodisperse solid particle in a colloidal suspension orvarious types of other challenge particles such as oleic acid orpotassium chloride (KCl). Non-monodisperse particles may be filtered toa monodisperse and known size by various techniques known independentlyin the art (such as through a differential mobility analyzer, notshown). Alternatively, non-monodisperse particles may be generated andfed into the filter chamber test section 551. The particle counters ofthe filter chamber test section 551, discussed below, may then simplysize the generated particles. Particles generated by the nebulizer arethen dried by the diffusion dryer 507. The diffusion dryer 507 may be,for example, a tube for air and particle flow surrounded by a desiccantdrying material. Excess charge on the particles produced by thenebulization process is removed by the charge neutralizer 509, therebyreducing agglomeration of the particles. In some embodiments, the chargeneutralizer 509 may include a chamber with one or more Kr-85 or Po-210beta-emitter sources.

The filter chamber test section 551 includes an upstream opticalparticle counter 513A and a downstream optical particle counter 513B.The optical particle counters can be any of a variety of particlecounters known independently in the art including, for example, a laserparticle counter or a condensation nucleus counter. The upstream opticalparticle counter 513A samples particles produced by the particlegeneration section 531 and entering the filter chamber test section 551through an upstream sampling probe 511A. A filter placed in the filterholder 517 filters at least a portion of the generated particles.Particles penetrating through the filter are sampled by downstreamsampling probe 511B in direct fluid communication with the downstreamoptical particle counter 513B. Certain types of particle counters canalso be used to size upstream and downstream particles.

Using data collected from the exemplary test apparatus 500, a fractionalefficiency of the filter under test can be calculated by the followingequation

$F_{eff} = {{\frac{C_{up} - C_{down}}{C_{up}} \cdot 100}\%}$where F_(eff) is the fractional efficiency of the filter as a functionof particle size (in microns), C_(up) is the concentration per unitvolume of particles collected upstream of the filter under test, andC_(down) is the concentration per unit volume of particles collecteddownstream of the filter under test.

FIG. 6 is an exemplary graph 600 showing fractional efficiency as afunction of particle diameter in testing the combination filter 431 ofFIG. 4B. The graph plots efficiency 603 of the filter under test (fromFIG. 5) for a number of different particle diameters 601. The lowestefficiency indicated by the underlying data used to produce theexemplary graph 600 is 99.99%. Thus, the combination filter 431 (FIG.4B) passes the stringent requirements established both under NESHAP andthe State of California HEPA filter requirement for challenge particles.

In addition to filter efficiency, NESHAP also requires that a filter betested for pressure drop as the filter becomes loaded with paint. In afurther test conducted by LMS Technologies, Inc., paint was fed intosample filters to determine pressure drop as a function of mass of paintfed. Table I, below, describes parameters used to produce the pressuredrop test on a test filter.

TABLE I FILTER DESCRIPTION 20″ × 20″ FILTER CONSISTING OF 1″ POLY PADFIRST STAGE, POLY PANEL SECOND-STAGE, AND SIX POCKET BAG COMBINATIONFILTER THIRD-STAGE PAINT MATERIAL DEFT TWO PART 44-GN-72 CHROMATECORROSION INHIBITOR PAINT SPRAY CONVENTIONAL AIR GUN AT 45 PSIG METHODSPRAY FEED RATE 106 GRAMS PER MINUTE; 100 CM³ PER MINUTE AIR VELOCITY120 FEET PER MINUTE FACE VELOCITY THROUGH FILTER

Table II, below, presents results from the pressure drop test regardingspray removal efficiency and paint holding capacity of the test filter.

TABLE II INITIAL PRESSURE DROP OF THE 0.355 INCHES OF WATER THREE-STAGEFILTER SYSTEM FINAL PRESSURE DROP OF THE 0.979 INCHES OF WATER LOADEDFILTER SYSTEM PAINT HOLDING CAPACITY OF 910 GRAMS FIRST FILTER INITIALREMOVAL EFFICIENCY 99.999% OF ALL THREE-STAGES (PAINT PENETRATION = 0.0GRAMS) AVERAGE REMOVAL EFFICIENCY 99.999% OF ALL THREE-STAGES (PAINTPENETRATION = 0.0 GRAMS) FIRST-STAGE PRESSURE DROP 0.650 INCHES OFWATER; AND GAIN IN MASS 910 GRAMS SECOND-STAGE PRESSURE DROP 1.07 INCHESOF WATER; AND GAIN IN MASS 730 GRAMS THIRD-STAGE PRESSURE DROP 1.26INCHES OF WATER; AND GAIN IN MASS 509 GRAMS

FIG. 7 is an exemplary graph 700 showing pressure drop as a function ofmass of paint in testing the combination filter 431 of FIG. 4B. Plot 703indicates pressure drop as a function of mass of paint fed on thefirst-stage filter, plot 709 indicates pressure drop as a function ofmass of paint fed on the second-stage filter, and plot 713 indicatespressure drop as a function of mass of paint fed on the third-stagefilter. The final point indicated on each of these three plotscorrelates with the tabular results of Table II. The final point on eachof these three plots also provides a point when each of the individualfilters is ready for replacement.

Being closest to the output of the spray gun, the first-stage filterloaded with paint the fastest. Thus, after the first-stage filter wasfully loaded, the first-stage filter was removed at point 705 in thetest with over 2800 grams of paint being fed (corresponding to the finalloading of 910 grams of overspray loaded onto the first-stage filter).At that point in the test, plot 701 indicates that the total pressuredrop measured across all three filters was less than 1 inch of water.After the second-stage filter was fully loaded, the second-stage filterwas removed at point 711 in the test with over 5800 grams of paint beingfed (corresponding to the final loading of 730 grams loaded onto thesecond-stage filter). At that point in the test, plot 707 indicates thatthe total pressure drop measured across the last two stages of thefilter system was over 1.4 inches of water. The third-stage filter wasfully loaded at point 715 in the test with over 7600 total grams ofpaint being fed (corresponding to the final loading of 509 grams loadedonto the filter). At that point in the test, plot 713 indicates that thetotal pressure drop measured across the final filter was approximately1.26 inches of water.

With reference to FIG. 8, an exemplary method 800 for constructing thecombination filter 431 of FIG. 4B is shown. In operation 801, a firstlayer media for the third-stage of the three-stage filter is cut tolength. The first layer media may be, for example, the Kimberly Clark 95SP media, described above. The second layer HEPA media for thethird-stage is cut to approximately the same length as the first layerin operation 803. The second layer media may be, for example, theHollingsworth & Vose TECHNOSTAT® media, described above. Each of thefirst layer media and the second layer HEPA media are interleaved (e.g.,stacked layers with each layer consisting of the first layer media andthe second layer HEPA media) in operation 805. The layer of interleavedmedia is then formed into a pocket in operation 807 with the first layermedia arranged inside of the second layer HEPA media. That is, in anexemplary embodiment, the first layer media is arranged to be upstreamof the second layer HEPA media once the final version of the combinationfilter 431 (FIG. 4B) is formed and placed into the filter wall 100 ofFIG. 1. The pocket may be formed by adding a single fold to the twolayers of media (i.e., the interleaved first and second layers) suchthat the first layer media is inside the second layer HEPA media afterthe fold is made. A cross-sectional view of the interleaved media isshown and discussed in more detail with reference to FIG. 9B, below.Sides of the pocket are then sealed, sewn, glued, or otherwise adheredin operation 809. Sealing the sides of the pocket is performed toprevent airflow from escaping out the sides of the formed pocket. A sideopposite the fold is left open to conduct airflow. As will be readilyunderstandable to a skilled artisan, the formed pocket can be likened toa pillow case or other bag with the open portion of the pillow case orbag eventually positioned in the completed filter to face toward theincoming airflow.

In operation 811, a mouth is formed on the open side of the pocket toaccept a rigid or semi-rigid device to keep the pocket open. One suchdevice is a metallic J-channel frame, formed into a rectangle, to beplaced within the mouth of the formed to pocket so as to keep the formedpocket open to an incoming airflow. J-channels are known independentlyin the art. Other types and shapes of channels are also knownindependently in the art and may be formed from plastic or other rigidor semi-rigid materials. The J-channel frame is inserted into the mouthformed on the pocket opening in operation 813. The J-channel frame isshown and discussed in more detail with reference to FIG. 9B, below

A number of pocket assemblies (i.e., the two-layer pocket with theJ-channel frame inserted into the mouth of each pocket) may then beinserted, adjacent and parallel to one another, into a header assemblyin operation 815. The header assembly may be formed as a square orrectangle from a U-channel arranged such that the open portion of the“U” faces inward to accept the J-channel frames. The U-channel may beformed from materials similar or dissimilar from the J-channel. Also,although this exemplary embodiment describes a U-channel for readyunderstanding of the concepts described, a number of other cross-sectionother than a “U” shape may be utilized including flat, square, round, orother geometrical shapes.

Each of the adjacent pocket assemblies are then stapled, glued, orotherwise adhered to one another and to the header assembly. Since theJ-channel frame is inserted into the pocket, an upper edge of the formedmouth and J-channel frame of each pocket presses against an adjoiningformed mouth and J-channel frame to prevent airflow from being conductedbetween the individual pocket assemblies (i.e., adjacent pockets willnot allow air to pass therebetween). In a specific exemplary embodiment,six pocket assemblies are used in a header assembly to complete thecombination filter 431 of FIG. 4B.

FIG. 9A shows a front elevational view 901 of a combination filterconstructed in accordance with the method of FIG. 8. The frontelevational view is shown to include a header assembly 903, a firstrectangular J-channel frame 905, a second adjoining rectangularJ-channel frame 907, and a number of formed pockets 911A. Each of theformed pockets 911A is individually coupled to a respective rectangularJ-channel frame.

In a specific exemplary embodiment, the header assembly 903 is a square,metallic frame with outside dimensions of about 50.8 cm by 50.8 cm by1.9 (approximately 20 inches by 20 inches by 0.75 inches). Each of theJ-channel frames is about 50 cm by 7.3 cm by 1.6 (approximately 19.75inches by 2.75 inches by 0.625 inches). These dimensions are exemplaryonly and other dimensions and numbers of J-channel frames may be used.

FIG. 9B shows an example cross-sectional view of one of a pocketassembly 911B of the combination filter of FIG. 9A. The pocket assembly911B comprises a first media layer 915A and a second layer HEPA medialayer 915B. A direction of airflow is indicated by an arrow 913 showingthe airflow entering the mouth 917 of the pocket assembly 911B.

In an example, the formed pockets 911 of FIG. 9A may be formed indifferent ways. For example, with reference to FIG. 9C, a rear view of asix inflated pockets 931 are shown. The inflated pockets 931 occur whenairflow is directed through a pocket due to the air resistanceencountered by the airflow through each of the pockets. An airflowindicator 935 shows the direction of airflow. In FIG. 9C, the directionof the airflow is out of the paper (i.e., toward the viewer). Each ofthe six inflated pockets 931 cause adjacent ones of the pockets to touchone another, forming a contact region 933 between the adjacent pockets.The contact region 933 restricts airflow. Therefore, much of the surfacearea of the pocket is blocked by the adjacent pocket (including pocketsfrom adjacent filters (not shown), both above and below the six inflatedpockets 931. Consequently, since adjacent pockets restrict the airflowdue to the contact region 933, most of the airflow exiting the pocketscan only be exhausted through an end (i.e., downstream) portion of thepocket.

With reference to FIG. 9D, a rear view of six modified pockets 951 isshown. In FIG. 9E, a side view of the six modified pockets 951 is shown(although only one of the six pockets can be seen from a side view).Also, in FIG. 9D, the airflow indicator 935 shows the airflow coming outof the paper, as with FIG. 9C. In FIG. 9E, the airflow indicator 935shows the airflow coming from the left into the six modified pockets951.

With reference to FIGS. 9D and 9E concurrently, a bonded portion 955 oneach of the six modified pockets 951 reduces a likelihood of adjacentpockets touching one another. The bonded portion 955 is formed on adownstream region of each of the six modified pockets 951 and formedsubstantially parallel to a direction of the airflow. A reduced contactregion 953 is seen to leave more open area between adjacent pockets thanthe contact region 933 of FIG. 9C. Since the contact region is reduced,airflow can exhaust through at least portions of the side of each pocketas well as the respective ends of the pockets. Consequently, thepressure drop across the pockets (in parallel to the airflow) is lessthan the six inflated pockets 931 of FIG. 9C due to the increased areathrough which airflow may be exhausted. As shown in FIG. 9D, at least aportion of the six modified pockets 951 is shown touching an adjacentpocket. However, depending upon a length and a number of bonded portionsused, the pockets may be arranged to not touch during filter operation,thereby further reducing airflow restrictions. In other examples, thebonded portion 955 may only be applied to certain ones of the pockets.For example, the bonded portion 955 may only be applied to every otherpocket.

The bonded portion 955 is formed by, for example, sewing the pockets soas to secure a first face of the second layer HEPA media to an opposingside of the second layer HEPA media (i.e., from a proximal side to adistal side of the pocket). Alternatively, a radio-frequency (RF)sealing or sonic welding process may be used. In a specific exemplaryembodiment, a length (i.e., from the open portion of the pocket to anopposing end portion, defining a depth of the pocket) of the bondedportion 955 may be from about 10 cm (approximately 4 inches) to about 15cm (approximately 6 inches). Depending on depth of the pocket, thebonded portion 955 may be shorter or longer and may be expressed as apercentage of the pocket depth. For example, the bonded portion 955 maybe from about 25% to 75% of the pocket depth. An exact percentage willbe at least partially dependent on an opening width of the pocket on theopen end or mouth of the pocket—if the opening width is narrower, thebonded portion 955 can extend closer to the open end without restrictinginlet airflow by not allowing the pocket to open to its full width. Inother embodiments, two of more bonded portion may be formed on eachpocket. The number of bonded portion formed on each pocket will be atleast partially dependent upon a size of the filter. For example, alarger filter (e.g., 61 cm by 61 cm or approximately 24 inches by 24inches) may be formed with pockets approximately 61 cm across. A smallerfilter of 50.8 cm square may be formed with pockets of approximately50.8 cm across. The larger 61 cm wide pockets can accommodate and maybenefit (e.g., in reduced pressure drop) from more bonded portion thanthe smaller 50.8 cm pockets.

FIGS. 10A and 10B show exemplary alternative fabrication techniques forconstructing the filter of FIG. 9A. For example, rather than forming thepockets as a single-folded unit as discussed with reference to theexemplary method 800 of FIG. 8, a single first layer media pocket 1003of FIG. 10A can be formed to fit within a single second layer HEPA mediapocket 1001. By forming each of the two layers of media separately,either media can be replaced, in the field, independent of the other.For example, if one or more of the single first layer media pocket 1003becomes clogged with paint (perhaps due to a proximity to a paint spraygun), the media can be changed out independently without having toreplace the entire filter.

A height, H−h₁, of the single first layer media pocket 1003 may beslightly less than a height. H, of the single second layer HEPA mediapocket 1001 to allow easy insertion of the first pocket into the secondpocket. Similarly, a length L₂, of the single first layer media pocket1003 may be slightly less than a length, L₁, of the single second layerHEPA media pocket 1001. After inserting the first pocket into the secondpocket, each of the media pockets may be adhered to one another inmanners similar to those described above such as by, for example,stapling, gluing, RF sealing, sonic welding, or a number of othertechniques, known independently in the art.

With reference now to FIG. 10B, an array of first layer media pockets1007 may be constructed to be inserted into an array of second layermedia pockets 1005. As with the single pockets just discussed, a skilledartisan will readily understand how to determine width and lengthdimensions to allow one unit to be inserted in the other. Also, anend-user can simply order replacement ones of the first layer mediapockets 1007 or the array of second layer media pockets 1005 simplybased on stating a first width W₁ and a second width W₂ of the filtersize. Although not shown in either FIG. 10A or 10B, a skilled artisan,upon reading the disclosure presented herein, will realize that afterthe single first layer media pocket 1003 or the array of first layermedia pockets 1007 is inserted into the respective pockets, a bondedportion, discussed above with reference to FIG. 9E, may be added to oneor more of the pockets.

As discussed herein, various rules govern filter testing andrequirements for paint overspray arrestance. Many of these test andrules are specific for a given industry. For example, paint oversprayarrestance in the aerospace industry may be more stringent than otherindustries due to the use of, for example, chromated paints. Also, therules can vary by governmental enforcement. As noted, the Federal Rulesin the United States for paint overspray arrestance are less stringentthan rules in the State of California directed to the same purpose.

One of the benefits of various embodiments of the combination filterdiscussed herein is that the pressure drop or resistance to air flow issubstantially lower than a HEPA filter. Further, the combination filterdiscussed herein combines the third and fourth stages of a filterdesigned to comply with rules in the State of California. Therefore, thevarious embodiments of the can be readily retrofit into existing filterwall systems without need to rebuild structures or increase fan size andmotor horsepower requirements. Further, filter replacement costs arelowered using the various embodiments of the combination filterdiscussed herein because the filter is less expensive and weighs lessthan would be required by a standard third- and fourth-stage filtercurrently used.

The above description and the drawings illustrate some embodiments ofthe inventive subject matter to enable those skilled in the art topractice the various embodiments described. Other embodiments mayincorporate structural, process, or other changes. Examples merelytypify possible variations. Portions and features of some embodimentsmay be included in, or substituted for, those of others. Many otherembodiments will be apparent to those of ordinary skill in the art uponreading and understanding the description provided herein. Consequently,in the foregoing Detailed Description, a skilled artisan may recognizethat various features may be grouped together in a single embodiment forthe purpose of streamlining the disclosure. This method of disclosure isnot to be interpreted as limiting the claims. Thus, the following claimsare hereby incorporated into the Detailed Description, with each claimstanding on its own as a separate embodiment.

What is claimed is:
 1. A single-stage combination filter, comprising: afirst layer of the single-stage combination filter having a firstplurality of media pockets arranged in a first array and arranged inparallel to receive an input airflow; and a second layer of thesingle-stage combination filter having a second plurality of mediapockets arranged in a second array and arranged in parallel to receive adischarge from the first layer, at least one of the pluralities of thefirst plurality of media pockets and the second plurality of mediapockets including at least one bonded portion formed on side edges ofrespective ones of the media pockets, the bonded portion to reduce alikelihood of adjacent ones of the media pockets touching one another.2. The single-stage combination filter of claim 1, wherein the at leastone bonded portion is formed on a downstream region of the mediapockets.
 3. The single-stage combination filter of claim 1, wherein theat least one bonded portion is configured to be substantially parallelto a direction of the input airflow.
 4. The single-stage combinationfilter of claim 1, wherein each of the first plurality of media pocketsis formed as an individual single pocket insert configured to be placedwithin a corresponding one of the second plurality of media pockets. 5.The single-stage combination filter of claim 1, wherein the firstplurality of media pockets is formed as an array of pocket insertsconfigured to be placed within a corresponding array of the secondplurality of media pockets.
 6. The single-stage combination filter ofclaim 1, wherein each of the first plurality of media pockets and thesecond plurality of media pockets is formed as individual pockets foldedwith edges of each of the pockets sealed to one another.
 7. Thesingle-stage combination filter of claim 1, wherein the first pluralityof media pockets has an efficiency less than the second plurality ofmedia pockets.
 8. The single-stage combination filter of claim 1,wherein the single-stage combination filter has a form-factor to fitinto a standard filter wall.
 9. The single-stage combination filter ofclaim 1, wherein the single-stage combination filter is configured tohave an initial pressure drop of about 60 Pascal.
 10. The single-stagecombination filter of claim 1, wherein the single-stage combinationfilter is configured to have a final pressure drop of about 300 Pascalprior to at least one of the first plurality of media pockets and thesecond plurality of media pockets being replaced.